State-of-the-art lithium-ion cells have a lithiated carbon negative electrode, or anode, (LixC6) and a lithium-cobalt-oxide positive electrode, or cathode, Li1−xCoO2. During charge and discharge of the cells, lithium ions are transported between the two host structures of the anode and cathode with the simultaneous oxidation or reduction of the host electrodes, respectively. When graphite is used as the anode, the voltage of the cell is approximately 4 V. The LiCoO2 cathode, which has a layered structure, is expensive and becomes unstable at low lithium content, i.e., when cells reach an overcharged state at x≧0.5. Alternative, less expensive electrode materials that are isostructural with LiCoO2, such as LiNi0.8Co0.2O2, LiNi0.5Mn0.5O2 and LiMn0.33Ni0.33Co0.33O2 are being developed with the hope of replacing at least part of the cobalt component of the electrode. However, all these layered structures, when extensively delithiated become unstable, because of the high oxygen activity at the surface of the particles. Therefore, the delithiated electrode particles tend to react with the organic solvents of the electrolyte or lose oxygen. Such reactions at the surface of layered lithium metal oxide electrodes are detrimental to the performance of the lithium cells and batteries, and methods are required to combat these reactions to ensure that maximum capacity and cycle life can be obtained from the cells.
Considerable efforts have already been made in the past to overcome the stability and solubility problems associated with layered LiCoO2 and LiNiO2 electrodes. For example, considerable success has been achieved in the past by stabilizing these electrodes by pre-treating the electrode powders with oxide additives such as Al2O3 or ZrO2 obtained from metal alkoxide precursors such as solutions containing aluminum ethylhexanoate diisopropoxide (Al(OOC8H15)(OC3H7)2 or zirconium ethylhexanoisopropoxide (Zr[(OOC8H15)2(OCH3H7)2]) as described, for example, by J. Cho et al in Chemistry of Materials, Volume 12, page 3788 (2000) and J. Cho et al in Electrochemical and Solid State Letters, Volume 4 No. 10, page A159 (2001), respectively, or a zirconium oxide, polymeric precursor or zirconium oxynitrate (ZrO(NO3)2.xH2O) as described by Z. Chen et al in Electrochemical and Solid State Letters, Volume 5, No. 10, page A213 (2002), prior to the fabrication of the final electrode thereby making the surface of the LiCoO2 particles more resistant to electrolyte attack, cobalt dissolution or oxygen loss effects.
The loss of oxygen from lithium metal oxide electrodes, such as layered LiCoO2 and LiNi1−yCoyO2 electrodes can contribute to exothermic reactions with the electrolyte and with the lithiated carbon negative electrode, and subsequently to thermal runaway if the temperature of the cell reaches a critical value. Although some success has been achieved in the past to improve the performance of lithium-ion cells by coating electrode particles, the coatings can themselves impede lithium diffusion in and out of the layered electrode structure during electrochemical discharge and charge. Further improvements in the composition of layered lithium-metal oxide electrodes, particularly at the surface of the electrodes, and in methods to manufacture them are still required to improve the overall performance and safety of lithium cells.